For onion topping:
1 1/2-2 tablespoons unsalted butter
1-2 tablespoon olive oil
2-3 large onions, cut lengthwise into 12 wedges
1 teaspoon fresh thyme leaves
1/2 teaspoon salt
1/4 teaspoon black pepper
3 oz coarsely grated Gruyère (3/4 cup)
2 cups all-purpose flour
2 teaspoons baking powder
1 teaspoon baking soda
3/4 teaspoon salt
2 oz coarsely grated Gruyère (1/2 cup)
3/4 stick (6 tablespoons) cold unsalted butter, cut into pieces
3/4 cup well-shaken buttermilk
3/4 teaspoon dry mustard
Preheat oven to 400°F.
Make onion topping: Heat butter and oil in a 12-inch heavy skillet over moderate heat until foam subsides, then cook onion, stirring occasionally, until golden brown, about 25 minutes. Remove from heat and stir in thyme, salt, and pepper, then cool. Toss cheese with onion and spread in bottom of a 9 1/2-inch deep-dish glass pie plate.
Make crust: Blend flour, baking powder and soda, and salt in a food processor. Add cheese and pulse 3 or 4 times to combine. Add butter and pulse until mixture resembles coarse meal.
Whisk together buttermilk and dry mustard in a small bowl and add to processor, then pulse until liquid is incorporated and dough begins to clump (do not let it form a ball).
Turn dough out onto a lightly floured surface and fold it over on itself several times to lightly knead. Form dough into a ball and let stand, covered with plastic wrap, at room temperature 10 minutes.
Roll out dough between 2 sheets of plastic wrap into a 10-inch round. Remove top sheet of plastic wrap and flip dough over onto onion. Peel off remaining plastic and tuck dough inside rim of pie plate.
Bake pie in middle of oven until crust is golden brown and firm to the touch, 25 to 30 minutes. Cool pie on a rack 2 minutes, then invert a platter over pie and flip pie onto platter. Serve warm.
Source: Gourmet, 2001